Pre-recorded or self-recorded optical discs may support “Out Of Multiplex” (OOM) formats. Out of multiplex is a format that stores different presentation components, e.g. video, audio and subtitles, on different locations on the disc, i.e. different files. This may be possible with various standardized media, e.g. Blu-ray disc. Also, a video technique known as multi-angle, or seamless angle change, may be implemented. It is well known from DVD-Video, and it means that a video film may contain multiple video tracks that are parallel on the time-line, wherein each video track usually shows the same scene from a different perspective. The user may select a perspective, e.g. by pressing a dedicated button. It is a requirement that switching between the different video tracks is performed with minimal delay, and that the tracks are integrated seamlessly into the video.
During playback of OOM formats, a number of the presentation components need to be decoded separately, and then presented synchronously and continuously, i.e. without noticeable interruption of the presentation. Therefore it is required that each of the decoders is provided with a continuous stream of data. On the other hand, optical drives are usually equipped only with a single pick-up unit, which is capable of reading only one data stream from the disc at a time, at a higher data rate than required by any of the decoders. That means that the pick-up has to jump from stream to stream in order to serve all decoders simultaneously. This requires buffering, i.e. the data that are read from the disc are fed into properly sized buffers, wherein each of the buffers is associated with a decoder. Buffering the coded data is more effective than buffering the decoded data. Thus, any decoder is provided with a continuous data stream.
Feeding the buffers requires a scheduling algorithm, because any buffer under-run must be prevented for a continuous presentation. The buffer sizes strongly depend on the applied scheduling scheme.
A pick-up usually contains an actuator carrying an optical sensor, and the pick-up is movable by a mechanical drive for raw adjustment, while the actuator is separately movable for fine adjustment without a mechanical drive. When the pick-up has to read multiple streams, this requires a much higher pick-up jump frequency than a single, multiplexed stream.